Free space optical terminal with dither based alignment

A bidirectional free space optical communications system comprising:

• two terminals communicating by free space optical (FSO) communications links, each terminal acting as a transmitter for one of the FSO communications links and as a receiver for the other of the FSO communications links;
• wherein each FSO communications link comprises:
• a beam steering unit (BSU) in the transmitter that steers a data-encoded FSO beam in a direction towards the receiver;
• a motion generator coupled to the BSU that controls the BSU to dither the direction of at least a portion of the transmitted FSO beam;
• a detector in the receiver that detects a received power of at least the portion of the transmitted FSO beam;
• and
• a feedback loop from the detector in the receiver to the BSU in the transmitter, the feedback loop including a return data-encoded FSO beam from the receiver to the transmitter, the feedback loop controlling the direction of the transmitted FSO beam based on the received power of at least the portion of the transmitted FSO beam,
• wherein the motion generator in one of the terminals dithers at a frequency f1 and the motion generator in the other of the terminals dithers at a frequency f2, and misalignment of the transmitted FSO beams is determined by demodulating the received powers using frequencies f1 and f2 and time averaging over a time period that is an integer number of cycles of the frequencies f1 and f2.

A bidirectional free space optical communications system comprising:

• two terminals communicating by free space optical (FSO) communications links, each terminal acting as a transmitter for one of the FSO communications links and as a receiver for the other of the FSO communications links;
• wherein each FSO communications link comprises:
• a beam steering unit (BSU) in the transmitter that steers a data-encoded FSO beam in a direction towards the receiver;
• a motion generator coupled to the BSU that controls the BSU to dither the direction of at least a portion of the transmitted FSO beam, wherein:
• the motion generator dithers the direction of the at least the portion of the transmitted FSO beam in two dimensions, and
• the motion generator dithers the direction of at least the portion of the transmitted FSO beam in a circular scan of amplitude Ad and frequency f, and alignment errors X10 and Y10 in the X- and Y-directions of the transmitted FSO beam are described by:
• X
• 1
• ⁢
• 0
• =
• R
• g
• ⁢
• 1
• 2
• ⁢
• A
• d
• ⁢
• 〈
• log
• ⁡
• (
• P
• 2
• ⁡
• (
• t
• )
• )
• ×
• cos
• ⁡
• (
• ω
• 1
• ⁢
• t
• )
• 〉
• Y
• 1
• ⁢
• 0
• =
• R
• g
• ⁢
• 1
• 2
• ⁢
• A
• d
• ⁢
• 〈
• log
• ⁡
• (
• P
• 2
• ⁡
• (
• t
• )
• )
• ×
• sin
• ⁡
• (
• ω
• 1
• ⁢
• t
• )
• 〉
• where Rg1 is a beam radius of the transmitted FSO beam that overfills an aperture of the receiver, P2 is the received power of the transmitted FSO beam, ω1=2πf1, and angle brackets ( ) indicate time-average over multiple cycles at rate ω1;
• a detector in the receiver that detects a received power of at least the portion of the transmitted FSO beam;
• and
• a feedback loop from the detector in the receiver to the BSU in the transmitter, the feedback loop including a return data-encoded FSO beam from the receiver to the transmitter, the feedback loop controlling the direction of the transmitted FSO beam based on the received power of at least the portion of the transmitted FSO beam.